The present invention relates to improvements in gears especially designed for chain drives and to a method for making the same.
Gears of the type to which the present invention relate are not new per se. U.S. Pat. No. 3,076,352 shows a gear having a ring plate embedded in the gear body wherein the ring plate is provided at its outer radial edge with a collar which supports the toothed rim section. It has been found that the compressive and bending strengths of the teeth of the gear depend essentially on the strength of the casting material used which may be plastic. Since the teeth of many gears, particularly gears for chain drives are relatively long, they are subjected to relatively large bending stresses and therefore the gear body of presently known gears must be cast from an extremely strong and relatively expensive plastic material. These high-strength plastic casting materials are usually very hard and relatively complicated to process since there is always the danger of fractures due to the brittleness of the material. Furthermore, a toothed rim section produced from hard casting material produces undesirable noise during operation of the gear.
To this end, the present invention provides a gear particularly adapted for chain drives comprising a gear body cast of a relatively soft, shock-absorbing plastic material, the gear body having a hub section, a gear rim section with spur teeth and a web section joining the hub section and gear rim section. An annular ring plate made of metal is embedded in the web section. The outer peripheral edge of the stiffening ring plate is located adjacent the land areas between the spur teeth and has a series of circumferentially spaced axially directed openings filled with the casting material about its periphery to lock the two in place after the casting operation. The ring plate is provided at its outer radial edge with a series of circumferentially spaced radially outwardly projecting fingers which are disposed in each case in radial alignment with the spur teeth and which are embedded in the casting material of the gear body. The finger-like projections extend beyond the bottom of the land areas between the spur teeth and located in radial alignment therewith is an axially directed opening machined in the ring plate. This construction provides a gear body capable of absorbing shock notwithstanding high compressive and bending stresses of the spur teeth.
Further, the individual teeth of the toothed rim section are stiffened both in the peripheral and axial direction by the finger-like projections. The toothed flanks which are stiffened by the projections have high compressive strength even if the gear body is made of a resilient shock-absorbing plastic. Thus, in spite of the resilience of the casting material, each tooth of the toothed rim section has high bending strength which means that the gear can withstand severe operating conditions which subject the teeth to high loads.
In accordance with another feature of the present invention, the area between adjacent finger-like projections and the outer edge of the ring plate matches the contour of the land spaces or flanks between the spur teeth. This provides particularly high stiffness and strength at the bottom of the land areas which is advantageous particularly in gears of chain drives which are subjected to considerable stress at these locations. The finger-like projections of the metal ring plate preferably extend for at least half the radial height of the spur teeth to ensure that the tooth flanks near the root where they are subjected to the greatest loads are supported by the outer edge of the ring plate and are, therefore, relatively stiff and unyielding. Moreover, in the area of the tip of the teeth, the tooth flanks are designed to be increasingly elastically resilient as a function of the compressibility and/or bending elasticity of the casting material used. In this fashion, the outer tips of the teeth guide the individual chain links radially inward to the bottom of the land areas and minimize start-up jolts while simultaneously aligning the links.
In accordance with still another feature of the present invention, the ring plate has at its inner peripheral edge at least one axially extending collar confronting and engaging the outer periphery of the outer ring of the bearing to provide a firm support for the gear with the result that the toothed rim is supported rigidly and concentrically in the radial direction on the bearing. Therefore, the teeth of the rim gear body engage with a chain or with an associated counter gear at the prescribed pitch circle notwithstanding the fact that the casting material of the gear body has a degree of elastic resilience. The ring plates with the collar section may be produced rather economically by a stamping or pressing operation from a semi-finished flat sheet of metal strip material.
In accordance with a modified embodiment of the present invention, the metallic ring plate may be provided with an axial slot extending radially inwardly from the axial openings disposed below the finger-like projections and in this manner the webbed section of the gear body has a degree of flexibility in a circumferential direction. The webbed section and hub section can also be made somewhat flexible.
In accordance with still another feature of the present invention, the ring plate is provided at its inner edge with a circumferential radial recess which expands towards the axial end of the collar sections and which is filled with an elastic or plastically resilient casting material of the gear body. This ensures that the surface pressure between the bearing assembly and the sides of the collar section embracing the bearing are approximately equal in the axial direction even though the ring plate has greater radial stiffness at its radial inner edge near the webbed section. This prevents bulging of the ring plate when it is seated on the bearing element which may be harmful.
This invention also provides a novel method for making gears having the features described above. To this end the gear body and ring plate with axial openings formed therein are formed by introducing the casting material uniformly into a mold from the periphery on one side of the ring plate whereby the material flows over the outer edge of the ring plate and also through all of the axial openings to the other side of the ring plate. In this manner, the finger-like projections are anchored very solidly in the casting material against coming loose. Further, by this process, casting streams develop which are decelerated at the edge of the ring plate and in the holes. On the opposite side of the ring plate, the casting streams meet each other so that seams occur in the plastic at these locations. These seams have a pattern which is rotationally symmetrical to the axis of the gear. By reason of forming the gear in this fashion, the spur teeth have high bending strength since the contact surfaces of the teeth have no seams which may weaken the casting material. It has been found that seams of an injection molded plastic are especially harmful if it is a fiber reinforced plastic since the strength of the material is reduced at the location of these seams by reason of unfavorable fiber orientation. The present invention overcomes these difficulties when utilizing fiber reinforced plastic inasmuch as the seams are not formed or produced at the highly stressed portions of the gear due to the manner in which the casting streams develop during molding as described above.